Optimized conditions for the assay of clinically useful enzymes have for the most part been based, in the past, on empirical information. These empirically-based enzyme assays offer the prospective user very little basis for evaluation the merits of one set of conditions as compared to another set of conditions. The proposed work is to establish and then use the kinetic properties of enzymes of clinical interest as the basis for optimal conditions. The primary objective is to establish and then use the kinetic properties (i.e., the substrate Michaelis constants, the theoretical maximal velocity and the initial velocity equation) of human gamma-glutamyltransferase isoenzymes as the basis for optimal conditions for assay of this enzyme in biological fluids.